The program calculates the source term activity as a function of time for parent isotopes as well as daughters. Also, at each time, the "probable release" is produced. Finally, the program determines the time integrated probable release for each isotope over the time period of interest.

The ACT-ARA program modifies an initial source term for changes with time due to decay of parents and production of daughters. The calculated activity for each daughter is based on the activity of the chain's parent. Time steps are distributed uniformly on a base 10 logarithmic scale. Decay constants, half-lives, specific activities, identified daughter products and their rate of production are maintained in data files IS30, IS50, IS60, IS90, and IS106. The probable release of the activity is calculated at each time interval based on the fraction released and the probability of intrusion. The fraction released is a step function which changes from an initial release fraction to a final release fraction at a specified time. The probability of release is non-zero following a period of active control and is modelled by a "Wisconsin Regression" function. The integrated probable release for each isotope is obtained by summation of the probable release over the total time.